1. Field of the Invention
This invention relates to a semiconductor device of very excellent reliability and a method for producing the same, and more particularly to a semiconductor device free of any trouble due to alpha-rays for a package and a method for producing the same.
2. Brief Description of the Prior Art
In general, semiconductor devices are sealed usually by the ceramic package (including also a method -cerdip- in which the ceramic package is sealed with glass), the plastic package, or the like. Especially in the ceramic package among these packages, ceramic materials contain uranium, thorium etc. on the order of several ppm. As stated in, for example, the "16th Proceedings of Reliability Physics (1978)" at page 33, it has been known that these impurities emit alpha-rays and therefore cause failures (called "soft errors") in memory devices etc. For this reason, the reliability of the semiconductor devices may lower conspicuously.
On the other hand, when powder, such as alumina, called filler is used in plastic package materials, the impurities are also contained in the filler. The filler is less influential on the integrated circuit devices than in the case of the ceramic encapsulation because it is surrounded with plastic materials such as epoxy resin and the alpha-rays emitted from the impurities are absorbed by the plastics. These plastic materials, however, have the disadvantages of comparatively low reliability in moisture resistance and heat resistance on account of the fact that the moisture absorbability is high, the fact that the contents of alkali impurities typified by Na are high and the fact that the heat resistance is as low as at most about 150.degree. C. It is therefore difficult to employ plastic-encapsulated integrated circuit devices in systems of high reliability. Accordingly, integrated circuit devices for use in the high-reliability systems need to be subjected to the ceramic type encapsulation.
For these reasons it has been strongly desired to prevent soft error of the semiconductor memory device caused by alpha-particles coming from uranium and thorium contained in the ceramic package as impurities.
In Electronics, June 8, 1978, pages 42-43, it is shown that this problem is attacking from several different directions, for instance, by devising new packaging material process or applying protective coating to the upper surface of the chip.
However, this article does not disclose the most favorable materials or necessary characteristics of the protective coating in order to prevent soft error caused by alpha particles.
IEEE Journal of Solid State Circuits, vol. SC-13, No. 4, August, 1978, pages 462-467 shows planar multilevel interconnection technology employing a polyimide resin.
However, in this article, the polyimide films having a thickness of 2.5 .mu.m are employed for interlevel dielectrics and final passivation. It must be recognized, however, that such thin films of polyimide cannot prevent penetration of alpha particles coming from outside of the film such as the ceramic package.
The use of the polyimide film as the protective film or coating to prevent soft error of the semiconductor memory device caused by alpha particles is not disclosed in this article.
Also, some prior art references show the use of polyimide resin in the field of the semiconductor devices, however, none of these references show the use of polyimide as a protective film or coating to prevent soft error of a semiconductor memory device caused by alpha particles.
For instance, U.S. Pat. No. 4,017,886 provides a polyimide layer between an SiO.sub.2 layer and a metal layer to bond a wire with the electrode very easily by forming a flat upper surface on which the metal layer is formed.
Furthermore, Japanese Patent publications No. 47-12609 and No. 52-26989 show the uses of polyimide for insulation and final passivation, respectively.
There is no disclosure in those prior art references concerning prevention of soft error caused by alpha particles.